The observational data on flare stars observed in star clusters and associations as well as in the solar vicinity (the UV Ceti type stars) are discussed. The analysis of these data show that they constitute one common class of objects possessing flare activity and the differences between them are conditioned by the age differences. The stage of flare activity is an evolutionary stage, one of the earliest stages of evolution passed by all red dwarf stars. It comes before the end of their T Tau stage of evolution. The UV Ceti type flare stars in the solar vicinity seem to be the population of the general galactic field, which were formed in the systems, already desintegrated. Most probably the stellar flares are the result of the release of the the surplus energy having intra-stellar origin.

Time resolved U-filter lightcurves of stellar flares on the dM4e star AD Leo have been measured to determine the flare frequency for individual observing seasons between 1972 and 1988. This report adds new data for 1984-1988. We use the number of flares with energy larger than 10∗∗30 erg, and the number of flare maxima with amplitudes larger than 0.3 as separate parameters to estimate flare frequencies. Chi-square tests do not reveal statistically significant variations of the flare frequency with time for 1972-1988.

During 9 nights in 1986-1987 we have carried out the patrol observations of red dwarf EV Lac within the wavelength range from 3500 Å to 8 mm and registered more than 50 optical flares. Some features of stellar flares, spots and active regions for the periods of cooperative observations are discussed.

Results of the patrol observations of the EV Lac flares in U, Hα and Hβ bands are given. The observations are carried out simultaneously in U and Hα or in U and Hβ. During (U, Hα) observations 21 U -flares are registered of which only comparatively strong one is accompanied by a real Hα - flare. And during (U, Hβ) observations among 7 U-flares the strongest and a “slow” one is accompanied with an increase of Hβ -signal. Small amplitude periodic light variations of EV Lac are detected with a period of 3d. 3 and amplitude 0m.02 in B and V bands.

Photoelectric U - band observations of the flare stars A Leo and EV Lac during the last 9 years obtained at the Bulgarian National Astronomical Observatory revealed 8 rapid spike flares on AD Leo and 9 rapid spike flares on EV Lac which duration is less than 6 seconds. The corresponding total monitoring time is 173.6 hours for AD Leo and 173.3 hours for EV Lac.

Synchronous observations of the UV Cet flares were carried out in 1987. Two 60-cm telescopes in U-band were used. Accuracy of syncronous registration on two telescopes is 0.001s. 15 flares were registered simultaneously during observations. Single short-time (up to 2s) light increases were observed in quiescent state and flares, which are the result of observation errors. No light increase with duration shorter than 10s was registered simultaneously on two telescopes.

Observations of flare stars with high time resolution permit to register spiky type flares and also to study in detail bright curves of flares. In Byurakan observatory observations of EV Lac have been made with 0.1 s time resolution simultaneouslyin two filters “U” and “B” [1-3]. These observations permited to detect flares of a burst type (Fig.1) and also to reveil multipeak structure of long duration flares. It is necessary to stress that simultaneous observations in two colours increas appresiably the thrustworthiness of the registered spiky flares.

Flare events on a time scale of the order of one second were observed on EV Lac by Gershberg and Petrov [1], Zalinian and Tovmassian [2], Tsvetkov, Antov and Tsvetkova [3]. The nature of such phenomena is very mysterious. We hope that monitoring of EV Lac with high time resolution will yield the information about growing and decay times as well as fine structure of the light curve and color one. A short-lived flare of EV Lac was recorded in U and V bands in 1989, September 3 2h 14m 30s UT. The duration of the event was about 150 milliseconds.

Diagnostic possibilities of the Walraven photometric system to analyze the UV Cet type flare stars have been studied. It was shown that the Walrawen system marks out the hydrogen spectrum features better than the standard UBV system. Observations of the UV Cet flare of 1985 December 23, carried out in the Walraven system were analyzed with computed two-colour diagrams, temperature and gaseous density at the maximum of brightness were estimated.

Photoelectric observations of flares on the two red dwarfs Gliese 171.2A and CM Dra are reported.

Photoelectric monitoring of flare stars YZ CMi, AD Leo, and EV Lac has been done at the Okayama Observatory since early 1970s. This is a simultaneous UBV observations with a high time resolution. Some statistics of the flares of the UV Ceti type stars are presented.

Current stellar models predict fully convective structures for masses less than about 0.3 Mo. We have monitored low luminosity dMe and dM stars for flare activity with the 2.1 m telescope at McDonald Observatory. The dMe stars produced 2-5 detectable U-filter flares per hour while none were seen in the dM stars. Typically, 15-30% of the U-filter flux received from the dMe stars is due to flares. The flare activity level of dMe stars (on an absolute scale) decreases as one approaches the lower end of the main sequence. Non-linear frequency distributions of flare energy are seen in some binary flare stars, suggesting that both stars contribute to the ensemble of flares observed from the binary. Upper limits for the dM stars suggest activity levels about one thousand times smaller than in the dMe stars.

The observational data permit us to establish statistical correlations between different parameters of stellar flare activity and the characteristics of quiet stars. We discusss physical consequences following from these relationships.

The results of a search for slow light variations of 13 K2e-M4.5e dwarf stars using the Sternberg Institute plate collection are presented. These variations are discovered for PZ Mon and V577 Mon, confirmed for BY Dra and BD+26°730 and suspected for V639 Her and V654 Her.

The mathematical expectation for detection of stellar flare on UV Ceti type stars in the solar vicinity during photographic patrol observations with 40″ Shcmidt camera of the Byurakan Astrophysical Observatory is estimated. We use the luminosity function of the flaring red dwarfs the assume a uniform distribution in the general galactic field. Comparison with the results of photographic patrol supports this assumption. The numbers and total mass of the flare and non-flare red dwarf stars in the Galaxy for the uniform distribution are determined. They are not in contradiction with Oort's estimate of total mass of red dwarfs.

In 1984-1986 new observations of flare stars in the Pleiades region were carried out by the method of stellar tracks with 40″ Schmidt telescope of the Byurakan observatory. The effective observational time is 100 hours in U-rays (ORWO ZU-21 plates combined with UG-1 filter were used). The stellar tracks have 1-1.5 cm length, the limiting stellar magnitude on the plates is 13m-14m, time resolution is 5S-20s. 17 flares of 13 stars were detected. Six of these stars were detected as flare stars for the first time. According to the Hertzsprung catalogue all these stars are the Pleiades cluster members. The flare amplitudes reach up to 1.5-3.0 magnitudes, the duration vary from 2.5 to 10.5 seconds.

With the aid of the star track method we discovered a possible new, relatively bright, flare star in the Pleiades region.

Photometric data on flares of cluster flare stars published by different observers were - as a rule - collected during decades by different instruments and amidst various conditions. The uncritical use of these data in global statistical studies is usually unfair since such a collection is an indefinite mixture of high and low quality results. In order to make these investigations more reliable even the best and most complete photometric catalogues have to be filtered.

The first step of the procedure described in the paper is rejecting all flare events having too small amplitudes - seeing that they are under-represented in the catalogue as opposed to medium and large amplitude flares. Then the photographic coverage of the aggregate has to be checked and all events observed in partially covered regions have to be excluded. Next any non-randomness in the time distribution of flares of each active object is to be unveiled and flares of the doubtful stars have to be omitted. Finally some objects are to be rejected for their individual characteristics.

The usefulness of the method is demonstrated with the example of the Pleiades, one of the richest open clusters known. More than 3000 hours of effective observing time has been devoted to flare photometry in fields centered upon Eta Tauri and the outcome of the programme the CPFS - the most complete catalogue of flare stars of the region - contains data of 519 flare stars and their 1532 flare events. Earlier versions of this catalogue or subsets of its data have been used to estimate the probable number of flare stars in the Pleiades region. Results of these attempts range from 600 to 2700 partly owing to the unreliability and incompatibility of the data sets. The method presented here is able to give much more reliable estimates but of course in the limited largest common field (LCF) only. As a result of the filtering the LCF contains 77% of the objects and 67% of the flares listed in the CPFS. Time sequence analysis of the 1026 flare events observed on 402 flare stars and statistical modelling of the phenomena suggested in an earlier paper fit each other almost perfectly.

The conclusion of the study is that the most likely number of flare stars in the reach of the telescopes used in this programme is between 520 and 600 in the LCF of the Eta Tauri fields. Their average flare frequency is about 1/2400h and the CPFS lists 70% of these objects. In order to be able to compare the Pleiades field flare stars and those of other aggregates the method of filtering will be carried out throughout the analysis of the most extended data sets available.